I've a TA908e with 10.3.2 and I've enabled ip sip proxy duplicates-allowed and this allows my phones on the Voice VLAN to get restrations now able to reach my SBC however I'm finding that shared lines on multiple endpoints are able to be active at the same time, only the most recently registered will take inbound calls and I would like all endpoints to be able to get the invite. Any suggestions on how to get SIP Forked registrations working on this device?

So, I got an answer to my question from support but to answer yours, we're using Metaswitch as our softswitch and ACME NetNet's for our SBC's. What I noticed was that the contact URI was oddly proprietary in syntax wherease it basically had the following sip:<number>@<ipaddr>;adtnpxyid-123kjdfl-jdbdf and it's the semicolon in the Contact URI that's throwing it off. So adding the following statement to my adtran config 'ip sip proxy grammar proxy-id contact-user changed the URI syntax to sip::<number>adtnpxyid-123kjdfl-jdbdf@<ipaddr> and now all my endpoints that are trying to register with the shared line appearance behind my 908e are coming up and processing calls as planned. Pressing hold and sharing the line between endpoints is indeed now working.

I have to say documentation is not good and there are no real good Adtran supported or created guides that walk through the practicality of this statement and why it's necessary. Might wanna add something in the future to your documentation. I could have resolved this issue in my SBC, however I'd like to try and make the fix as far out to the edge as possible and not add addtl complexity to my 14+ regional SBC's..

Thanks for everything tho.. I do like Forum formats and can find answers much more quickly.. hopefully my fix will assist someone else.

That's why I was asking what softswitch you were using. The proxy ID is used internally by the Adtran unit to keep track of endpoints in the proxy table. Both options should be within the specifications for SIP, but the Metaswitch is the only softswitch that requires that specific syntax (SIP grammar command).

Thank you for spelling out the resolution on here for others to see. I'll mark this thread as assumed answered, but feel free to post additional questions.